1. Field of the invention
The present invention relates to a chemical amplification resist copolymer sensitive to radiation and a resist composition containing it. More particularly, the present invention relates to a radiation-sensitive copolymer useful as a matrix resin to form micro patterns, and a chemical amplification resist composition based on the copolymer, which makes it possible to perform a sub-micro lithography process using a deep UV such as a KrF excimer laser or an ArF excimer laser, an X ray such as synchrotron radiation, or charged particle beams such as electron beams.
2. Description of the Prior Art
A lithography process used in fabricating semiconductor devices generally comprises coating a resist film on a substrate such as a silicon wafer, exposing the coating to light, and developing it to form a positive or negative pattern. Certainly, the recent thrust into the high integration of semiconductor devices is greatly based on advances in microlithography. For example, ultra-fine patterns as small as sub-microns, e.g. 0.2 microns or less, in size, are required for the fabrication of ultra-LSI. Now, the light sources used to form the fine patterns become increasingly shorter in wavelength, for example, from g-line or I-line, to deep UV light, including a KrF excimer laser and an ArF excimer laser, further to an X-ray, and finally to an electron beam.
With near UV light, such as I-line, which is used in conventional lithography, micro patterns as small as sub-quarter microns (0.25 microns) are virtually impossible to realize. Such a micro pattern requires shorter wavelengths which belong to deep UV excimer lasers, X rays, and electron beams. Of them, KrF and ArF excimer lasers occupied the attention of the researchers in expressing such exquisiteness, and were developed as a light source, requiring novel photoresists. Now, chemical amplification photoresists are prevalently used for deep UV light.
A chemical amplification resist composition suitable for deep UV light fundamentally comprises a polymer with an acid-dissociable functional group, a compound which generates an acid (hereinafter referred to as xe2x80x9cphotoacid generatorxe2x80x9d), and a solvent, and avails itself of chemical amplification effect in lithography.
Japanese Pat. Laid-Open Publication No. Heisei 2-27,660 suggests a chemical amplification resist composition which is based on a mixture of a polymer containing a carbonylic acid t-butylester or phenyl t-butylcarbonate group and a photoacid generator. This composition utilizes the fact that, when being irradiated, the acid generated by the photoacid generator dissociates the t-butylester or t-butylcarbonate group from the main chain and the hydroxy group thus formed allows the exposed area to be easily dissolved by an alkaline developing solution.
Most of the chemical amplification resists utilizing KrF excimer lasers are based on phenolic resins. They are, however, unsuited to ArF excimer lasers because their aromatic rings show large absorption peaks at the wavelength of the light source (193 nm).
Thus, there is a strong demand for a material which little absorbs the light belonging to such wavelength ranges. In response to the demand, active research has been directed to the development of the chemical amplification photoresists based on polyacrylate derivatives (Japanese Pat. Laid-Open Publication No. Heisei 4-226,461; Proc. SPIE, 1996, vol. 2724, p377).
Polyacrylate derivatives show little absorbance at 193 nm, but suffer from a great disadvantage in that they are far inferior in dry etch resistance. Recently, much effort has been made to overcome this disadvantage, including the introduction of alicyclic derivatives into polyacrylate. The introduction of alicyclic derivatives certainly brings about an improvement in dry etch resistance, but causes a significant problem in a developing process because their hydrophobicity has a negative influence on the affinity for developing solutions. In forming 0.2 micron or less patterns, the adherence of a resist composition to a substrate plays an important role. Carboxylic acid was introduced into the side chains of matrix polymers for the purpose of increasing the adherence (Proc. SPIE, 1997, vol. 3049, p. 126). Carboxylic acid-grafted matrix polymers, however, require a change in the basicity of developing solutions because the carboxylic acid increases the solubility of the matrix polymers in the aqueous alkaline solutions.
It is known that copolymers of maleic anhydride and olefin can be used as matrix resins which show not only hydrophilicity, but also etch resistance (Proc. SPIE, 1997, vol. 3049, p126). In the copolymers, maleic anhydride, responsible for hydrophilicity, serves as a promoter which enables the copolymerization with olefinic monomers to be accomplished at low temperatures at low pressures.
During the development of a base resin for photoresist, carboxyl-containing norbornene derivative monomers and maleic anhydride monomers give a great contribution to their polymers in improving adherence to substrate, transparency to deep UV light, and dry etch resistance as well as photosensitivity, resolution and developability.
Therefore, it is an object of the present invention to provide a copolymer as a base resin for photoresist, with which there can be obtained sufficiently fine patterns for the high integration of semiconductor devices by using deep UV light, such as a KrF excimer laser and an ArF excimer laser.
It is another object of the present invention to provide a chemical amplification photoresist composition consisting essentially of the resin and a photoacid generator.
In order to avoid the problems that conventional chemical amplification resists have in adherence to substrate, dry etch resistance and developability, a cyclic structure consisting of a maleic anhydride and norbornene derivatives is introduced into the main chain of the present invention with hydroxy groups and acid-dissociable functional groups being grafted. The cyclic structure improves dry etch resistance while the hydroxy group is responsible for increasing the adherence to substrate. The acid-dissociable functional groups take charge of improving photosensitivity and resolution. Particularly, the great advance in the perpendicularity of resist patterns and the sensitivity of resist patterns the present invention achieved, is attributed to the carboxylic acid-containing norbornene derivative introduced.
Accordingly, the present invention pertains to a polymer consisting essentially of maleic anhydride, a carboxylic acid-containing norbornene derivative, and an acid-dissociable functional group-containing norbornene derivative and to a chemical amplification positive resist composition sensitive to radiation.
More details are given of the present invention, below.
The polymer useful in the present invention has a repeat unit consisting of a norbornene derivative with a carboxylic acid grafted, a norbornene derivative with an acid-dissociable functional group grafted, and a maleic anhydride. The polymer itself is insoluble or hard-soluble in aqueous alkaline solutions and contains at least one protecting group which can be dissociated by acid.
The alkali solubility of the polymer is primarily determined by the content of the acid functional groups which are dissociable by acid. Accordingly, the resist properties of the polymer, including adherence to substrate, photo-sensitivity, resolution and the like, are dependent on the kind and quantity of the norbornene derivatives used in the polymer.
The polymer of the present invention is a multi-membered copolymer represented by the following general formula I: 
wherein, X is an acid-dissociable grafted norbornene derivative selected from the group consisting of the following general formulas II and III; Y is a carboxylic acid-grafted norbornene derivative represented by the following formula IV: 
wherein R1 is selected from the group consisting of straight or branched alkyl groups, cyclic or polycyclic alkyl groups, alkyl carbonyl groups, branched alkyl carbonyl groups, and cyclic or polycyclic alkyl carbonyl groups, all containing 1-10 carbon atoms, which are exemplified by t-butyl oxycarbonyl, acetyl, cyclohexane carbonyl, adamantane carbonyl, bicyclo[2,2,1]heptane methyl carbonyl and the like; R2 is selected from the group consisting of hydrogen, straight alkyl oxycarbonyl, branched alkyl oxycarbonyl, alkoxy alkyl carbonyl, cyclic alkyl oxycarbonyl and polycyclic alkyl oxycarbonyl, all containing 1-10 carbon atoms, independently for X and Y; and R3 is selected from straight or branched alkyl and cyclic or polycyclic alkyl, all containing 1-10 carbon atoms, which are exemplified by methyl, ethyl, t-butyl, iso-propyl, adamantyl, bicyclo[2,2,1]heptane methyl and the like; and
l, m, n and o each are a repeating number not more than 0.5, satisfying the condition that l+m+n+o=1 and 0.4xe2x89xa6oxe2x89xa60.5. The resist properties, such as adherence to substrate, photosensitivity and resolution, are taken into account in determining the repeating numbers, l, m and n.
The monomers represented by the general formula II may be exemplified by 3-acetoxy-3-bicyclo[2,2,1]-hept-5-en-2-yl propionic acid t-butyl ester, cyclohexane carboxylic acid 1-bicyclo[2,2,1]hept-5-en-2-yl butoxy carbonyl-ethyl ester, and adamantan-1-carboxylic acid 1-bicyclo[2,2,1]-hept-5-en-2-yl-2-t-butoxycarbonyl-ethyl ester.
Concrete examples of the monomer represented by the general formula III include bicyclo[2,2,1]hept-5-en-2-carboxylic acid methoxymethyl ester, bicyclo[2,2,1]hept-5-en-2-carboxylic acid ethoxymethyl ester, bicyclo[2,2,1]hept-5-en-2-carboxylic acid bicyclo[2,2,1]hept-2-yl methoxymethyl ester, 3-bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid 2-t-butyl ester 3-methoxylmethyl ester, bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid bis-methoxymethyl ester, bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid bis-ethoxymethyl ester, bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid 2-bicyclo[2,2,1]hept-2-yl methyl ester 3-methoxymethyl ester, bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid 2-bicyclo[2,2,1]hept-2-yl methyl ester 3-ethoxymethyl ester, bicyclo[2,2,1]hept-5-en-2,3-carboxylic acid 2-(bicyclo[2,2,1]hept-2-yl methoxymethyl)ester 3-bicyclo[2, 2,1]hept-2-yl methyl ester, bicyclo[2,2, 1]hept-5-en-2, 3-dicarboxylic acid 2-methoxymethyl ester 3-(1,7, 7-trimethyl bicyclo[2,2,1]hept-2-yl)ester, bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid 2-ethoxy methyl ester 3-(1,7,7-trimethyl bicyclo[2,2,1]hept-2-yl)ester, and bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid 2-(bicyclo[2,2,1]hept-2-yl methoxy methyl)ester 3-(1,7,7-trimethyl bicyclo[2,2,1]hept-2-yl)ester.
Examples of the monomer represented by the general formula IV include bicyclo[2,2,1]hept-5-en-2-carboxylic acid, 3-bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid mono-t-butyl ester, 3-formyl bicyclo[2,2,1]hept-5-en-2-carboxylic acid bicyclo[2,2,1]hept-2-yl methyl ester, and bicyclo[2,2,1]hept-5-en-2,3-dicarboxylic acid mono-(1,7,7-trimethyl bicyclo[2,2,1]hept-2-1)ester.
The repeating unit of the Formula I can be prepared by polymerizing a norbornene derivative represented by the general formula II or III, a carboxylic acid-containing norbornene derivative represented by the following structural formula IV, a maleic anhydride of the following general formula V, and 3-bicyclo[2,2,1]hept-5-en-2-yl-3-hydroxy-propionic acid t-butyl ester (hereinafter referred to as xe2x80x9cBHPxe2x80x9d) represented by the following structural formula VI, in the presence of a polymerization catalyst: 
Account must be taken of the adherence to substrate, sensitivity and resolution when determining the amount of these monomers.
When the X and Y moieties reveal themselves in the general formula I, the repeat unit of the polymer is represented by the following general formulas VII to XII: 
wherein R1 is selected from the group consisting of straight or branched alkyl groups, cyclic or polycyclic alkyl groups, alkyl carbonyl groups, branched alkyl carbonyl groups, and cyclic or polycyclic alkyl carbonyl groups, all containing 1-10 carbon atoms, which are exemplified by t-butyl oxycarbonyl, acetyl, cyclohexane carbonyl, adamantane carbonyl, bicyclo[2,2,1]heptane methyl carbonyl and the like; R2, R3, R4 and R5 are independently selected from the group consisting of straight or branched alkyl and cyclic or polycyclic alkyl, all containing 1-10 carbon atoms, which are exemplified by methyl, ethyl, t-butyl, iso-propyl, adamantyl, bicyclo[2,2,1]heptane methyl and the like; and l, m, n and o each are a repeating number not more than 0.5, satisfying the condition that l+m+n+o=1 and 0.4xe2x89xa6oxe2x89xa60.5. The resist properties, such as adherence to substrate, photosensitivity and resolution, are oaken into account in determining the repeating numbers, l, m and n.
These multi-membered polymers may be in the form of a block copolymer, a random copolymer or a graft copolymer. Preferable is an alternating copolymer of maleic anhydride and the norbornene derivatives or a random copolymer therebetween. They may be prepared in conventional polymerization processes and, preferably by the action of a radical initiator. For his radical polymerization, an available initiator may be azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO), lauryl peroxide, azobisisocapronitrile, azobisisovaleronitrile, or t-butylhydroperoxide, but is not specifically limited to them. The polymerization of the monomers may be carried out in a manner of bulk polymerization, solution polymerization, suspension polymerization, bulk-suspension polymerization or emulsion polymerization. Examples of useful polymerization solvents include benzene, toluene, xylene, halogenobenzene, diethylether, tetrahydrofuran, acetates, esters, lactones, ketones, amides and mixtures thereof.
The temperature of the polymerization is dependent on the polymerization catalyst employed. For example, if azobisisobutyronitrile is used as a polymerization catalyst, the polymerization is preferably carried out at a temperature of about 60-90 xc2x0 C.
As for the molecular weight of the polymer prepared, it can be controlled by varying the amount of the polymerization initiator and the period of polymerization time. After completion of the polymerization, the monomer residues which remain unreacted in the reaction mixture, and by-products are preferably removed by solvent precipitation. The polymer of the Formula I preferably has a polystyrene-reduced average molecular weight (hereinafter abbreviated to xe2x80x9cMwxe2x80x9d) ranging from about 1,000 to 100,000 as measured by gel permeation chromatography (GPC), and more preferably from about 3,000 to 50,000 when taking into account the sensitivity, developability, coatability and thermal resistance which are required for a photoresist. For example, if the polymer has an Mw of less than 1,000, the resulting photoresist composition is very poor in coatability and developability. On the other hand, if the Mw exceeds 100,000, degradation occurs in sensitivity, resolution and developability. In molecular weight distribution, the polymer of the invention preferably ranges from 1.0 to 5.0 and more preferably from 1.0 to 2.0.
As mentioned above, the molecular weights and molecular weight distributions of the polymers obtained were measured by use of GPC in the model HLC-8020 manufactured by TOSHO, equipped with columns G2000HXL, G3000HXL and G4000HXL, eluting tetrahydrofuran at a flow rate of 1.0 ml/min at a column temperature of 40xc2x0 C. with a standard of mono-dispersed polystyrene.
In the present invention, only one or a mixture of the polymers obtained may be used for the resist.
Photoresist patterns as fine as 0.2 microns can be usefully formed by use of the polymers of the present invention. Therefore, the present invention also pertains to a chemical amplification photoresist composition comprising the polymer and a photoacid generator.
As the photoacid generator useful in the present invention, an onium salt, such as iodonium, sulfonium, phosphonium, diazonium and pyridonium, will suffice. Concrete, particularly effective, but non-limitative examples of the onium salt include triphenylsulfonium triflate, diphenyl(4-methylphenyl)sulfonium triflate, diphenyl(4-t-butylphenyl)sulfonium triflate, diphenyl(4-methoxyphenyl) sulfonium triflate, dibutyl(naphthalen-1-yl)sulfonium triflate, triphenylsulonium hexafluoroantimonate, diphenyliodonium triflate, diphenyliodonium methylbenzenesulfonate, bis(cyclohexylsulonyl)diazomethane, and bis(2,4-dimethylphenylsulfonyl)diazomethane. A halogen compound also can be used as the photoacid generator, which is exemplified by 1,1-bis(4-chlorophenyl)-2,2,2-tricholorethane, phenyl-bis(trichloromethyl)-s-triazine or naphthyl-bis(trichloromethyl)-s-triazine. Besides, diazoketone compounds, such as 1,3-diketo-2-diazo, diazobenzoquinone and diazonaphthoquinone, sulfonic compounds, sulfonic acid compounds, and nitrobenzyl compounds may be candidates for the photoacid generator. More preferable are the onium compounds and the diazoketone compounds. The photoacid generators may be used singly or in combinations.
In contrast to common photoacid generators, the onium salts represented by the following general formulas XIII and XIV not only serve as dissolution preventers in unexposed areas, but act to promote dissolution in exposed areas: 
wherein R1 and R2, which may be the same or different, each represents an alkyl or an aryl; R3 and R4, which may be the same or different, each represents a hydrogen atom, an alkyl or an alkoxy; and n is an integer of 0-14.
The photoacid generators are used at an amount of approximately 0.1-30 parts by weight based on 100 parts by weight of the solid content of the photoresist composition, and preferably at an amount of 0.3-10 parts by weight. They may be used alone or in mixture of at least two species.
In the present invention, if necessary, there may be used a compound which is decomposed by acid to promote the dissolution of the photoresist in a developing solution. As such an acid-decomposable and dissolution-promoting function group, t-butyl ester is most suitable. Therefore, alicyclic derivatives containing t-butyl ester may be recruited in the present invention. Upon formulation, they may be added at an amount of approximately 3-60 parts by weight based on 100 parts by weight of the solid content of the photoresist composition, and preferably approximately 5-40 parts by weight.
Optionally, the photoresist composition of the present invention may comprise additives, such as a surfactant, an azo compound, a halation inhibitor, an adhesive aid, a preservation stabilizer, an antifoaming agent and the like. As for surfactant, it may be exemplified by polyoxylauryl ether, polyoxystearyl ether, polyoxyethyleneoleyl ether, polyethyleneglycol dilaurylate, etc. The surfactant is preferably used at an amount of 2 parts by weight or less based on 100 parts by weight of the solid content of the photoresist composition.
To obtain a uniform and flat photoresist coating, the solvent used is required to show an appropriate evaporation rate and viscosity. Examples of such a solvent include ethyleneglycol monomethyl ether, ethyleneglycol monoethyl ether, ethyleneglycol monopropyl ether, methylcellosolve acetate, ethylcellosolve acetate, propyleneglycol monomethyl ether acetate, propyleneglycol monoethyl ether acetate, propyleneglycol monopropyl ether acetate, methylethyl ketone, cyclohexanone, methyl 2-hydroxypropionate, ethyl 2-hydroxypropionate, 2-heptanone, ethyl lactate, and gamma-butyrolacetone. They, if necessary, may be used, alone or in combinations. Depending on its physical properties, such as volatility and viscosity, the solvent is used at such an appropriate amount that a uniform and flat photoresist coating could be formed on a wafer.
A photoresist film is typically obtained by coating the photoresist solution on a wafer and drying it. After being filtered, the photoresist solution may be coated by use of a spin coating, flow coating or roll coating technique.
Then, selective irradiation on the photoresist film coated is performed to give fine patterns. The available radiation, although it is not specifically limited, may be UV light, such as I-line, deep UV light, such as KrF or ArF excimer lasers, X rays, or charged particle beams, such as electron beams, depending on the photoacid generator used. Following the radiation, a thermal treatment may be optionally done to improve the sensitivity of the film.
Generally, the formation of photoresist pattern is finally completed in a developing solution. Examples of the developing solution include aqueous solutions of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium silicate, sodium metasilicate, ammonia water, ethyl amine, n-propyl amine, triethyl amine, tetramethylammonium hydroxide and tetraethylammonium hydroxide with particular preference to tetramethylammonium hydroxide. If necessary, additives, such as surfactants and aqueous alcohols, may be added.